The prevalent paradigm in radio frequency (RF) electronic communications is to treat 1) the RF modulating element and 2) the RF antenna as totally separate and distinct system elements. As such, each is designed and generally defined independently according to “black box” level specifications and connected together in a functionally modular fashion, wherein the baseband data message signal interacts with the RF carrier in the RF modulating element to form a composite signal wholly independent of the characteristics of the RF antenna. The composite modulated RF carrier signal is then provided as a generic input to the RF antenna. This architecture is illustrated in FIG. 1, and represents essentially all existing applications of art.
There are several important aspects of existing approaches to modulation that rely on the architecture illustrated in FIG. 1. First, the modulator stage is a lossy system component, wherein some of the RF carrier signal power is used up in the modulation process. This loss must be overcome through additional re-amplification of the output of the core modulating element, a function that is often, but not always, included internal to the integrated circuit or sub-system comprising the modulator. The amplification needed to overcome the losses associated with existing modulation techniques requires additional system power supply consumption.
Secondly, the existing architecture class of FIG. 1 requires that the final amplification stage process the composite modulated signal directly as it amplifies the composite signal up to the desired transmit power level prior to being fed to the RF antenna. Linearity performance requirements are thereby imposed on the power amplification stage such that a failure to meet the linearity requirements will result in an inability to achieve some desired level of transmit modulation accuracy and thus to some desired level of wireless communications link performance. It is a common requirement that amplifiers used to transmit high-order amplitude and phase modulated signals be “backed off” from their maximum operating output level in order to meet transmit signal distortion requirements, further reducing the maximum output power available to existing systems.
FIG. 2A is a graphical illustration of an RF signal. A pure unmodulated RF signal is a sinusoidal electromagnetic wave that oscillates at a fixed frequency (f) over time (t) with a particular amplitude (A) and phase (φ). The phase is a property that indicates where in its oscillating cycle a signal will be at any given time and space (up, down, or in between). An unmodulated signal oscillates over time, but the phase of the signal is fixed.
Phase is measured in radian (or degrees) with respect to a reference signal. At 0 (0°), a signal overlaps a reference signal; at π (180°) a signal is opposite to a reference signal; at 2π (360°) they overlap again. Since a signal of fixed frequency can be defined by phase and amplitude alone, it is easy to represent as a fixed point on a circle, where the radius of the circle is the amplitude A and the angle to the point is the phase φ. FIG. 2B is a graphical illustration of the RF signals of FIG. 2A mapped to a circle. This circle lies on what is called the “complex plane.”
Any point in the complex plane, and hence any signal of arbitrary phase and amplitude, has sine and cosine components:Acos(2πft+φ)=Acos(φ)cos(2πft)−Asin(φ)sin(2πft)   [1]where Acos(2πft+φ) is a signal of arbitrary amplitude A and phase φ, Acos(φ) is the amplitude of the cosine component of the arbitrary signal, and Asin(φ) is the amplitude of the -sine component of the arbitrary signal.
These components may be represented as projections onto the x and y axis of the complex plane. In this representation, the x axis is the cosine component (sometime referred to as the “in-phase” component or “I”) and the y axis is the -sine component (sometime referred to as the “quadrature” component, or “Q”). Because the choice of time 0 is arbitrary, I and Q are used rather than cosine and -sine. Thus, I refers to a signal that is “in-phase” with a reference signal, and Q refers to a signal that is out of phase by 90° or “quadrature” with a reference signal.
A primary objective of RF communications is to communicate information. Information is imparted to radio waves by altering aspects of the waves over time. Data may be represented by altering a carrier's phase, amplitude, frequency and/or polarization.
In digital wireless communication, bits are transmitted as binary information: 1s and 0s. For example, by switching (modulating) a carrier wave back and forth between two opposite (180° apart) phase states, the 1 and 0 “bits” of information may be communicated one bit at a time. This switching occurs at a modulation frequency (for example, 1 million modulations per second), and each modulation segment over time is referred to as a symbol. This particular modulation is called Binary Phase Shift Keying (BPSK), and it transmits 1 bit per symbol. FIG. 3 is a graphical illustration of BPSK modulation.
Modulation is not limited to two states. For example, switching between four different states permits transmission of two bits per symbol. FIG. 4A is a graphical illustration of Quadrature Phase Shift Keying (QPSK) modulation. QPSK switches between four points on the complex plane. The complex plane may be divided into many different states to transmit more information per symbol, generally by varying phase or amplitude
Radio waves not only have amplitude and phase, they also have a direction and a polarization. The type of polarization used in a radio system depends on the application requirements. In the context of an electromagnetic radiator, polarization is defined as the instantaneous vector direction of the electric field of the propagating wave from the perspective of the transmit antenna. There are basically three types of polarization, linear, circular, and elliptical, illustrated in FIG. 4B. Linear and circular polarization may be viewed as special cases of elliptical polarization. In linear polarization, the electromagnetic wave propagating outward from the transmitting antenna exists (and varies in amplitude as a cosinusoid) along a single vector direction. For an elliptically polarized wave, the electric field vector rotates around the axis of propagation as a function of time, tracing out an ellipse as seen from behind. When both orthogonal components of an elliptical wave have the same peak amplitude, then the polarization is said to be circular. Circular and elliptical polarization may be right handed or left handed.
There is a class of technologies that utilize antennas to modulate a carrier are sometimes described as using “direct antenna modulation” techniques. These methods tend to focus on amplitude modulation only, and do not leverage the spatial aspects of the antennas. Other current research efforts that use the term “antenna modulation” do not encode information symbols on a transmitted signal, but are rather attempts to achieve an increase in the equivalent instantaneous impedance bandwidth of an antenna, which is otherwise used in a traditional fashion. While these other methods do modulate the antenna structure, the implications and benefits of employing the spatial aspects of antenna modulation are neither addressed nor realized.